Applications for communication network technology and continuously expanding and increasing, and it is frequently necessary to synchronise various distributed devices with a network. This may be achieved by providing accurate timing information to the devices, enabling their synchronisation to a timing signal. Example devices which may need to be synchronised to a network include connected appliances in home and industrial environments. Such appliances may communicate over a cellular network with application servers that manage and configure the appliances. Further examples of devices requiring synchronisation include small cell basestations, particularly residential small cells and femtocells. Small cells are miniature basestations having reduced power output compared to macro basestations, and providing coverage or capacity extension to a communication network over a limited geographical area. Accurate timing information is particularly important for small cells, either to support Time Division Duplexing (TDD) technology used in the network or to minimise interference between small cells or between small cells and macro cells.
Various solutions for frequency and time or phase synchronisation have been proposed and defined by standardisation bodies, including the International Telecommunication Union Telecommunication Standardisation Sector (ITU-T) G.826x and G.827x series of recommendations. Two main approaches have been defined in these solutions, the first based on delivering frequency synchronisation over the physical layer. Synchronous Ethernet (SyncE) is an example of such an approach, in which clock signals are transferred over the Ethernet Physical Layer. The second approach involves delivering timing information (either frequency alone or frequency and time) via packets. The Precision Time Protocol, defined in IEEE 1588, is an example of a packet based synchronisation approach. The solutions defined by the standards bodies assume implementation in well managed telecommunication networks in which for example every node in the synchronization chain is expected to fulfill very strict requirements in terms of synchronization, or in the case of transparent transport of packets, with very strict constraints in terms of packet delay variation.
Various combinations of synchronization techniques have been proposed in order to improve performance and reliability of time synchronization. In one example, G8271.1 and G.8273.2 define an architecture in which IEEE1588 is combined with Synchronous Ethernet. In this case Synchronous Ethernet is be used to improve stability and to provide enhanced Holdover when the IEEE1588 reference is lost. In another example, a new work item in ITU-T is proposing to combine IEEE1588 carried over a network that is not able to process IEEE1588 with a GNSS source available at a base station. This scheme is known as “Assisted Partial Timing Support” (APTS) and will be specified in G.8273.4. An example configuration for APTS is illustrated in FIG. 1. During normal operation, the accurate GNSS reference (for example provided by a GPS receiver) provides a synchronization reference and may also be used to “calibrate” the noisy IEEE1588 signal which may be impacted by asymmetries in the network. During limited periods of time (for example 24 hours) during which the GNSS signal is lost, synchronization is maintained using the IEEE1588 signal.
In some deployment situations, the synchronization solutions described above may not be appropriate or may be difficult to implement. For example, the application of the ITU-T synchronization solutions to a femtocell or residential small cell environment is highly complex, owing at least in part to the presence of public internet segments, which may be used when connecting small cells to a network, and which segments cannot provide synchronization support.
FIG. 2 illustrates an example residential small cell deployment, in which various distributed devices 2 connect to a telecommunications network via a residential small cell 4. In one example deployment scenario, residential small cells are designed to be as simple as possible, reducing their cost and physical complexity. In such scenarios, only the following layers of the access spectrum may reside on a residential small cell providing Long Term Evolution (LTE) radio access: PHY, MAC and RLC. Other layers, including PDCP and RRC may be centralized, resulting in a very simple small cell structure when compared to standard femtocells or macro basestations. Traffic to and from the residential small cell may be in the form of encrypted PDCP PDUs carrying both data and signaling radio bearers. The encrypted PDCP PDUs may be mapped via an algorithmic procedure onto a packet infrastructure implemented through Ethernet, IPv6 or similar. More specifically, the PDCP PDUs may be provided with a MAC/IPv6 address and then switched to the centralized function. Each residential small cell thus comprises the reduced basestation functionality described above together with a switch and G.fast/xDSL modem.
A specific synchronization solution has been developed for synchronizing small cells. This solution, known as Network Listening, Cellular network listen (CNL) or Over The Air (OTA) synchronization, has been developed for synchronizing TDD femtocells, but it can also be applied to Frequency Division Duplexing (FDD) basestations. Network listening is defined in 3GPP technical report TR 36.922, and the corresponding signaling messages are specified in technical standards TS 36.413 and TS 32.592. Using this technique, a residential small cell, or Home eNodeB (HeNB) derives its timing from another synchronized eNodeB (eNB) or HeNB, using the downlink transmission of surrounding cellular basestations to provide a synchronization source for the HeNB.
As discussed in the Small Cell Forum white paper: “Synchronization for LTE small cells”, December 2013, Network Listening requires implementing a small subset of UE functionality in the small cell, which functionality may be used to detect adjacent cells and determine their relative timebase frequency error. These adjacent cells may be intra-frequency, inter-frequency (including inter-band), or inter-RAT (Radio Access Technology). Network Listening is based on the premise that some cells have an accurate frequency source. It is therefore possible for a small cell to synchronize its timebase frequency clock to the timebase frequency of these adjacent cells and still meet its own frequency stability requirements. If implemented with particular care, Network Listening may also be used to achieve phase synchronization. However, in order to achieve this, the location of the cell providing the synchronization signal, as well as that of the small cell receiving the signal, must be known precisely, in order to enable the propagation delay between source cell and receiving cell to be calculated. FIG. 3 shows a representative illustration of Network Listening synchronization, with HeNB1 synchronizing to a signal received from a synchronized eNB. The source cell for the synchronization signal need not necessarily be a macro cell connected to the Global Navigation Satellite System (GNSS), as multi-hop HeNB to HeNB synchronization is envisaged using Network Listening.
Although Network Listening has been defined for femtocells in TR 36 922, some aspects of the solution remain poorly specified, and the solution may not be well integrated with overall network synchronisation solutions.
In one example issue, coverage problems may mean that a high quality reference signal for delivery over the radio link may not always be available. In addition, it is necessary to avoid a situation in which a group of small cells use Network Listening to synchronise to each other, so forming a timing loop and drifting together from the network timing signal. From an implementation point of view, significant additional operational cost is required to deliver phase synchronisation, as the exact location of the synchronising small cell and the source of the synchronisation signal must be known. Distances between the source cell and synchronising small cell may be outside the control of the network operator, and could result in additional complexity in achieving phase synchronisation.